JotYou.com : Track Your Friends

JotYou.com can help you track your friends when they are in a particular location. When you will send a message to your friends it will be delivered when they will pass through an area specified by you. You can also send a message to yourself to remind something.

The JotYou.com HowItWorks page quotes:

JotYou™ is location based messaging. Send a message to your friends so they get it when they arrive at school or the ballpark or the mall. Make up geo-games. Track a foot race or bicycle race. Stage a road rally with virtual checkpoints, and feed the directions as participants progress. Ever plan to pick up the milk on the way home from work, but drive right by the store and forget? Use JotYou™ to remind yourself as you pass by specified locations. The possibilities are endless.